Isomeric Schiff bases related by dual imino-group reversals

Two isomeric pairs of Schiff bases, N,N′-bis­(2-methoxy­benzyl­idene)-p-phenyl­enediamine, C₂₂H₂₀N₂O₂, (I), and 2,2′-dimeth­oxy-N,N-(p-phenyl­enedimethyl­ene)dianiline, C₂₂H₂₀N₂O₂, (II), and (E,E)-1,4-bis­(3-iodo­phen­yl)-2,3-diaza­buta-1,3-diene (alternative name: 3-iodo­benzaldehyde azine), C₁₄H₁₀I₂N₂, (III), and N,N′-bis­(3-iodo­phen­yl)ethylenedi­imine, C₁₄H₁₀I₂N₂ [JAYFEV; Cho, Moore & Wilson (2005). Acta Cryst. E61, o3773–o3774], differ pairwise only in the orientation of their imino linkages and in all four individual cases occupy inversion centers in the crystal, yet all four compounds are found to assume unique packing arrangements. Compounds (I) and (II) differ substantially in mol­ecular conformation, possessing angles between their ring planes of 12.10 (15) and 46.29 (9)°, respectively. Compound (III) and JAYFEV are similar to each other in conformation, with angles between their imino linkages and benzene rings of 11.57 (15) and 7.4 (3)°, respectively. The crystal structures are distinguished from each other by different packing motifs involving the functional groups. Inter­molecular contacts between meth­oxy groups define an R₂²(6) motif in (I) but a C(3) motif in (II). Inter­molecular contacts are of the I...I type in (III), but they are of the N...I type in JAYFEV.